1. Field of the Invention
The invention relates generally to the field of acoustic well logging. More specifically the invention relates to methods for processing data from acoustic well logging instruments in order to obtain acoustic velocities of subsurface formations.
2. Background Art
Acoustic well logging instruments known in the art include a “sonde” or similar housing structure that can be moved along the interior of a wellbore by means of an armored electrical cable, coiled tubing, drill pipe or similar conveyance. The sonde includes one or more acoustic transmitters that when actuated impart a pulse of acoustic energy into the fluid in the wellbore. The transmitters are typically magnetostrictive or piezoelectric transducers that change shape in response to application of an electrical current pulse to the transducer. The acoustic energy is typically, although not exclusively, in a frequency range of 8 to 30 kHz. The acoustic energy travels through the wellbore fluid and impacts the wall of the wellbore. Relative acoustic velocities of the wellbore fluid and typical subsurface rock formations through which the wellbore is drilled are such that the acoustic energy refracts and travels along the wellbore wall. The wall-traveling energy also refracts back into the wellbore. The re-refracted acoustic energy is detected by one or more acoustic receivers disposed at selected distances along the sonde from the transmitter. The distance along the sonde is typically chosen to provide a substantial travel path along the wellbore wall through the various subsurface rock formations.
Techniques for acoustic well logging known in the art prior to the development of computerized well logging recording systems, many of which carried over into earlier computer-implemented surface recording systems, included what are known as interval transit time determination. In such techniques, a timer is started at the time the acoustic transmitter is actuated. The timer is stopped when acoustic energy detected by the receiver exceeds a selected threshold. The time taken by the acoustic energy to travel from the transmitter to the receiver is related to the acoustic velocity of the wellbore fluid and the formations disposed between the transmitter and the receiver. Early in the development of acoustic well logging instruments, a second receiver was included on the sonde at a different axial spacing than the first receiver. A travel time was determined from the transmitter to each receiver. An “interval transit time” was determinable by subtracting one receiver's acoustic energy travel time from the other's. The “interval” is generally disposed axially between the receivers and is offset by an average refraction angle of the acoustic energy and the distance from the exterior of the sonde to the wellbore wall. Such interval travel time determination substantially eliminated the need to determine the acoustic velocity of the wellbore fluid, however the travel paths in the wellbore for each of the two receivers may be different as a result of irregularities (rugosity) in the wellbore wall and any tilt of the sonde in the wellbore.
The foregoing travel path problems were dealt with by an acoustic well logging instrument known as the borehole compensated (“BHC”) instrument. BHC instruments included transmitters disposed at opposite ends of the sonde. Two or four receivers were disposed longitudinally between the transmitters, such that each transmitter included two corresponding receivers spaced at the same selected distances (typically three and five feet) from the transmitter. Four-receiver sondes included two longitudinally offset pairs of receivers, each pair including a three foot spacing and a five foot spacing receiver corresponding to one of the transmitters. The pairs were offset by an amount corresponding to the expected refraction angle of the acoustic energy at the wellbore wall. The offset provided better longitudinal correspondence between the formations investigated by the two receiver pairs. In BHC acoustic well logging, an interval transit time is determined for each receiver pair (or the one receiver pair for both transmitters in two-receiver BHC instruments). Because the corresponding transmitters are on opposed sides of the investigated formation, variations in interval transit time caused by sonde tilt and wellbore rugosity are substantially canceled by averaging the two interval transit times.
Acoustic well logging was initially developed for estimating the fractional volume of pore space (“porosity”) in subsurface formations. Later, acoustic well logging was used to correlate surface reflection seismic surveys to the subsurface formations actually penetrated by wellbores. An important parameter for such correlation is the acoustic velocity measured all along the wellbore. In certain types of rock formations, he acoustic velocity can be reduced by interaction of the drilling fluid with such formations proximate the wellbore wall. Such velocity changes are relatively large in certain geologic areas, such as the United States Gulf of Mexico outer continental shelf. To deal with the problem of formation velocity alteration, so called “long spacing” acoustic well logging instruments were developed.
In principle, long spacing acoustic well logging instruments operate the same as BHC well logging instruments, at least with respect to determining interval transit time. The difference between long spacing acoustic well logging instruments and their BHC counterparts is the longitudinal spacing between the transmitters and receivers. A typical long spacing acoustic well logging instrument includes two transmitters at one end of the sonde spaced apart by two feet. A first receiver is disposed near the other end of the sonde at about eight feet from the nearer transmitter, and a second receiver is spaced two feet further therefrom along the sonde. The relatively long axial span traveled by the acoustic energy along the wellbore wall is believed to result in acoustic energy traveling faster in the unaltered formation immediately adjacent to the altered formations at the wellbore wall. Thus, first energy arrivals will more likely correspond to acoustic energy traveling through faster, unaltered formations.
Long spacing acoustic well logging instruments are designed asymmetrically, with transmitters at one end and receivers at the other, primarily to avoid making the sonde so long as to be impractical to use. It is known in the art to obtain the equivalent of BHC measurements from a long spacing acoustic well logging instrument by what is known as the “depth derived BHC” technique. In depth derived BHC acoustic analysis, an interval transit time is determined at a formation adjacent to the transmitters at a first time. By reciprocity, such interval transit time may be determined using signals from one receiver and two transmitters in the same manner as explained above for one transmitter and two receivers, namely, by subtracting a travel time from one transmitter to a selected receiver from the travel time of the other transmitter. The foregoing interval travel time is stored and is later averaged with the interval transit time determined between the receivers when the instrument has moved such that the receivers are adjacent to the same formation previously evaluated by the transmitters.
It has been observed that depth derived BHC techniques may not be adequate to account for certain wellbore conditions. There exists a need to derive better estimates of formation acoustic velocities using long spacing acoustic well logging instruments known in the art.